Various implementations of apparatus for measuring a property of a fluid present in a tube are known. They are utilized in the most diverse apparatus, for example in dialysis monitors, for measuring any properties of the fluid. To this end, they generally comprise a sensor, which is suitable for measuring the desired property. The measured properties of the fluid are, for instance, the temperature, the flow velocity, the pressure, the conductivity, etc.
However, measuring these properties of the fluid requires the sensor to be in direct contact with the fluid, which is generally established through a lateral access opening, i.e. a lateral opening in the tube. For this, the sensor is arranged on the tube in such a way that it protrudes at least partially into the tube through the lateral access opening, and is thus in direct contact with the fluid which flows substantially completely around it. This is necessary for temperature sensors, for example, which must be substantially entirely surrounded by the fluid, or which have the fluid flow substantially entirely around them, in order to record an exact temperature.
The sensor may also be disposed on the tube such that it protrudes only into the lateral access opening, but not into the tube itself, so that it is in contact with the fluid, but the fluid does not flow completely around it, or is not entirely surrounded by fluid. This is adequate, for example, for sensors with electrodes for measuring the inductance of the fluid.
However, owing to the geometry of the tube it is difficult to provide a lateral access opening that, on the one hand, allows for direct contact of the sensor with the fluid and, on the other hand, is reliably sealed from the surroundings. Apart from the requisite direct contact with the fluid, the sensor disposed on the tube also needs a connection to an evaluating unit or the like, in order to relay the measured values.
To resolve the sealing problems, German Patent Publication No. 35 08 570 suggests, for example, inserting a shut-off valve with a plug in a tube, the plug comprising a bore in the axis of rotation. A sensor is inserted in the bore and sealed with a sealing ring. Then, when for example as a result of the composition of the fluid, deposits build up with time on the sensor that is in direct contact with the fluid and impede the exact measurement of the required measured value, it is possible to replace the sensor without difficulty. To this end, the shut-off valve is set in its blocking position so that the fluid flow is interrupted and the sensor can be replaced without loss of fluid.
A disadvantage of this type of apparatus is that the tube must be completely severed during installation of the shut-off valve. Thus, after successful installation of the shut-off valve, additional seal surfaces with additional possible unsealed areas are created. Moreover, a shut-off valve entails an additional material expense in addition to the supplementary installation expense, thus creating additional costs.
An apparatus is known from German Patent Publication No. 41 01 549 for measuring temperatures in tubes, in which a bushing is welded laterally to a tube and forms a lateral access opening. A plug having a central bore is inserted in the bushing. Furthermore, a temperature sensor is screwed into the bushing and is pushed through the bore of the plug to the fluid, and protrudes into the fluid. The plug, which seals the temperature sensor from the fluid, is specifically worked on the side directed towards the fluid. This is intended to prevent the formation of gaps between the plug and the tube wall as breeding grounds for bacteria and other germs.
While this makes possible a reliable seal of the temperature sensor from the fluid, this seal also requires a larger amount of labor. Specifically, lateral access to the tube must be provided, a bushing must be welded to the lateral access or the tube, respectively, and the plug specifically worked on the side directed towards the fluid, before the temperature sensor can be screwed into the bushing. This creates supplementary costs in addition to the required additional material.
An apparatus for measuring the temperature of a fluid present in a tube is known from European Patent No. 413,198, in which a bushing is similarly welded laterally to a tube and forms a lateral access opening to the fluid. An extensively worked ball valve is screwed into the bushing. A temperature sensor is pushed through the ball valve in the latter's open position up to the fluid in the tube, and is sealed against the fluid by ring seals before the ball valve and after the ball valve. In this way it is possible to replace the temperature sensor without shutting off the fluid. However, numerous seal surfaces are necessary, which increases the risk of leaks. In addition, this known apparatus is also expensive in terms of material and labor and is therefore costly.
In view of this background it is therefore an object of the present invention to provide apparatus for measuring a property of a fluid present in a tube, with a sensor for measuring the property, which sensor is arranged on the tube and in direct contact with the fluid through a lateral access opening, that is simple and inexpensive to manufacture and in which the lateral access opening is reliably, simply and inexpensively sealed.